To produce the output stage of the high-frequency generator, it has already been proposed to use circuits inspired by amplifiers operating in class D. These amplifiers indeed make it possible to attain much higher efficiencies than linear amplifiers operating in class A or less linear amplifiers operating in class B, AB or C. A class D amplifier operates using active components (transistors) controlled in an “all-or-nothing” manner between an “on” state and an “off” state at a frequency known as a switching frequency; in the “on” state, a current flows towards an output load, in the “off” state it is interrupted. The level of amplification is determined by the duty cycle of the conduction time at each period of the switching frequency. The input signal of the power stage is therefore a modulated signal in terms of pulse width at the switching frequency. A low-pass filtering (able to be part of the output load) eliminates the switching frequency at the output so as to conserve only the amplified input signal.
For a high-frequency generator output stage, it is possible to operate on the principle of a class D amplifier but without eliminating the switching frequency at the output, so that the output of the stage is only a power signal (modulated or not) at the switching frequency.
High-frequency generator output stages comprising two switches have therefore already been proposed, the switches being transistors operating in an “all-or-nothing” manner, activated at the desired high frequency so as to be successively and non-simultaneously conducting, linked to a power supply by inductive windings which receive currents of periodically reversed direction. These windings are coupled to other windings connected to an output load. The load receives a high-frequency alternating current.
Among the design constraints on these output stages, there is the fact that it is preferentially desirable to use transistors of the same type, rather than transistors of complementary types, as switches, for example two NPN transistors or two NMOS transistors but not an NPN transistor and a PNP transistor, or a NMOS transistor and a PMOS transistor. The reason for this is that it is desirable that the transistors have the same characteristics; now, it is difficult to obtain very similar characteristics, in terms of impedance or recovery time at the moment of switching, if the transistors are not of the same type. Additionally, the circuit diagrams are more complex if transistors of different types are used, especially for the control functions of the latter, above all if a variable supply voltage must be used, which is sometimes necessary to ensure a variable output power.
High-frequency generator output stages using transistors of the same type controlled in an “all-or-nothing” manner at the desired high frequency have already been proposed. They can use a double-wound transformer as is the case in the patent publication US2003/0179044. However, a double-wound transformer is expensive as its characteristics must imperatively be adapted to a given application and thus such a component is not readily commercially available.
Other propositions have been made based on these principles, for example in the U.S. Pat. No. 4,647,867, U.S. Pat. No. 5,726,603, U.S. Pat. No. 3,714,597, with relatively complex structures.
The technical literature also describes various high-frequency class D amplification circuits, and notably:    M. Seo, J. Jeon, I. Jung, Y. Yang “A 13.56 MHz high-efficiency current mode class-D amplifier using a transmission line transformer and harmonic filter”. Proceedings of the Asia-Pacific Microwave Conference 2011;    F. H. Raab Switching transients in class-D RF power amplifiers, in HF radio systems and techniques, 7-10 Jul. 1997, conference publication No 411, IEEE, 1997;    Hermann Schreiber, 350 schémas HF de 10 kHz à 1 GHz [350 HF diagrams from 10 kHz to 1 GHz], page 267, Editions Radio 1990.
All of these diagrams use inductors known as common-mode inductors composed of two windings coiled around one and the same magnetic core. Some of the diagrams use two common-mode inductors associated in a Guanella balun (this association method will be clarified further on). They have drawbacks and notably the fact that a DC component of high value can flow in each common-mode inductor, risking the saturation of the magnetic cores. Additionally, these diagrams do not make it possible to adjust an internal impedance value of the output stage of the generator, whereas it is sometimes desirable to be able to choose the internal impedance value so as to match it to the load located downstream of the generator.